This invention relates generally to semiconductor chips electrically and mechanically connected to a substrate, particularly to flip-chip configurations.
Flip-chip technology is well known in the art. A semi-conductor chip having solder bumps formed on the active side of the semi-conductor chip is inverted and bonded to a substrate through the solder bumps by reflowing the solder. Structural solder joints are formed between the semi-conductor chip and the substrate to form the mechanical and electrical connections between the chip and substrate. A narrow gap is left between the semi-conductor chip and the substrate.
One obstacle to flip-chip technology when applied to polymer printed circuits is the unacceptably poor reliability of the solder joints due to the mismatch of the coefficients of thermal expansion between the chip, having a coefficient of thermal expansion of about 3 ppm/xc2x0 C., and the polymer substrate, e.g. epoxy-glass having a coefficient of thermal expansion of about 16 to 26 ppm/xc2x0 C., which causes stress build up in the solder joints. Because the structural solder joints are small, they are thus subject to failures. In the past, the solder joint integrity of flip-chip interconnects to a substrate has been enhanced by underfilling the volume between the chip and the substrate with an underfill encapsulant material comprised of a suitable polymer. The underfill material is typically dispensed around two adjacent sides of the semiconductor chip, then the underfill material slowly flows by capillary action to fill the gap between the chip and the substrate. The underfill material is then hardened by baking for an extended period. For the underfill encapsulant to be effective, it is important that it adhere well to the chip and the substrate to improve the solder joint integrity. Underfilling the chip with a subsequently cured encapsulant has been shown to reduce solder joint cracking caused by thermal expansion mismatch between the chip and the substrate. The cured encapsulant reduces the stresses, induced by differential expansion and contraction, on the solder joints.
The underfill process, however, makes the assembly of encapsulated flip-chip printed wire boards (PWB) a time consuming, labor intensive and expensive process with a number of uncertainties. To join the integrated circuit to the substrate, a flux, generally a no-clean, low residue flux, is placed on the chip or substrate. Then the integrated circuit is placed on the substrate. The assembly is subjected to a solder reflowing thermal cycle, soldering the chip to the substrate. The surface tension of the solder aids to self align the chip to the substrate terminals. After reflow, due to the close proximity of the chip to the substrate, removing flux residues from under the chip is such a difficult operation that it is generally not done. Therefore the flux residues are generally left in the space between the chip and the substrate. These residues are known to reduce the reliability and integrity of the encapsulant.
After reflow, underfill encapsulation of the chip generally follows. In the prior art, the polymers of choice for the underfill encapsulation have been epoxies, the coefficient of thermal expansion and moduli of the epoxies being adjusted with the addition of inorganic fillers. To achieve optimum reliability, a coefficient of thermal expansion in the vicinity of 25 ppm/xc2x0 C. is preferred and a modulus of 4 GPa or more. Since the preferred epoxies have coefficient of thermal expansions exceeding 80 ppm/xc2x0 C. and moduli of less than 4 GPa, the inorganic fillers selected generally have much lower coefficient of thermal expansions and much higher moduli so that in the aggregate, the epoxy-inorganic mixture is within the desired range.
The underfill encapsulation technique of the prior art has four principal disadvantages:
1. The reflowing of the solder bump and then underfilling and curing the encapsulant is a multi-step process that results in reduced production efficiency;
2. To underfill a flip-chip assembly takes too long because the material must flow through the tiny gap between the chip and the substrate;
3. The flux residues remaining in the gap reduce the adhesive and cohesive strengths of the underfill encapsulating adhesive, affecting the reliability of the assembly; and
4. As the size of chips increase, the limiting effect of capillary action becomes more critical and makes the encapsulation procedure more time consuming, more susceptible to void formation and to the separation of the polymer from the fillers during application.
Clearly, many improvements to this process are feasible to increase reliability, reduce the time required and decrease the likelihood of producing a void in the encapsulant while providing the required low coefficient of thermal expansion and high modulus.
Other prior art methods of encapsulating the chip have attempted to overcome the above limitations by applying the encapsulating resin through a hole in the substrate located near the center of the chip. After the soldering and cleaning operations, the encapsulating resin is forced through the hole and around the periphery of the chip to ensure complete coverage of the chip surface. This method suffers from the need to reserve an area in the center of the substrate that is free of circuitry in order to provide an unused space for the hole. It also does not eliminate the problems of entrapped air bubbles.
Another prior art method in U.S. Pat. No. 5,128,746 (Pennisi) teaches a method wherein an adhesive material including a fluxing agent is applied to the chip or substrate. The chip is positioned on the substrate and the solder bumps are reflowed. During the reflow step, the fluxing agent promotes wetting of the solder to the substrate metallization pattern and the adhesive material is cured, mechanically interconnecting and encapsulating the substrate to the component. The limitation of this technique is that in order for the molten solder to readily wet the substrate metallization and also to allow the solder, through surface tension, to self-align the chip bumps to the substrate metallization pattern, the material must maintain very low viscosity during the reflow step. But the viscosity of these materials is severely increased by the presence of the required inorganic fillers. As a result, this approach has failed to produce a material that can serve as both the flux and the encapsulant with the required low coefficient of thermal expansion and high modulus for optimum reliability.
Referring to FIGS. 1 and 2, underfilling the chip 100 with a subsequently hardened encapsulant 102 has been shown to reduce solder joint cracking caused by thermal expansion mismatch between the chip and the substrate 104. The hardened encapsulant 102 transfers the stresses, induced by differential expansion and contraction, from the solder joints 106 to deformation of the chip 100 and substrate 104 as shown in FIG. 1 for expansion-induced strain at elevated temperatures and FIG. 2 for contraction-induced strain at reduced temperatures. In other words, the main effect of the hardened encapsulant during thermal expansion or contraction is to effectively force the chip and the substrate to take up the stress caused by the coefficients of thermal expansion mismatch by bending and bulging the chip and substrate. This bending and bulging reduces the stress on the solder joints and virtually eliminates solder fatigue failure.
Unfortunately, the effect of the encapsulant bending the substrate and the chip causes its own new set of problems. One such problem is that the bending makes the chips susceptible to cracking. Another such problem is that the degree of stress relief is highly dependent on the flexibility of the under-lying substrate and is thus an unpredictable function of the design of the printed circuit. Another limitation is that relying on such bending for stress relief on the solder joints prevents the placement of flip chips directly opposite one another on a double-sided printed circuit.
Another limitation of prior art flip-chip attachment is the difficulty of performing rework. Chip removal, once underfill has been performed, is very destructive to both the printed circuit board and the chip. Rework is almost impossible with prior art materials and processes. For example, the prior art procedure for removing an encapsulated die from a printed wire board is to grind it off manually.
Another limitation of the prior art is the expense of applying solder bumps to a chip. The solder bumps have been applied to chips by one of several methods. Coating the solder on the chip bumps by evaporation of solder metals through a mask is one such method. This method suffers from 1) long deposition times, 2) limitations on the compositions of solder that can be applied to those metals that can be readily evaporated, and 3) evaporating the metals over large areas where the solder is ultimately not wanted. Also, since most solders contain lead, a toxic metal, evaporation involves removal and disposal of excess coated lead from equipment and masks. Another common method in the prior art is electroplating of the solder onto the chip pads through a temporary sacrificial mask. Electroplating is a slow and expensive process that also deposits the solder over large areas where the solder is ultimately not wanted. Another method is to screen print solder paste on the chips pad through a stencil, then reflowing the solder to form a ball or bump on the pad. This technique is limited to bump dimensions that can be readily stencil printed, so it is not practical in bump pitches of 25 microns or less.
Another limitation of the prior art is the difficulty in distributing electrical signals from the small dimension of the chip to the large dimensions of the substrates. Most chips are manufactured with the electrical interconnection pads around their periphery with a pad pitch of 0.25 mm or less. On the other hand, printed circuits are manufactured with pad pitches of 0.25 mm and larger. This discrepancy in dimensions requires that the chip-to-substrate interconnection provide some method of redistributing the chip pad locations over a larger area so that they can match the dimensions of the printed circuit. Today, this discrepancy is bridged by creating expensive redistribution layers on the printed circuit. Few manufacturers are able to produce printed circuits at the tight dimensional tolerances required for redistribution, but those who are capable of doing so achieve this with significant production yield penalties. Another method to bridge the dimension discrepancy involves complete redesign of the chip to redistribute the electrical pads over the entire area of the chip, an expensive procedure that chip manufacturers generally want to avoid.
In one aspect of the present invention there is provided a chip with underfilling encapsulant and separate discrete solder bumps pre-coated and pre-assembled on the chip for assembly to a substrate. This configuration provides a simple, cost-effective assembly procedure wherein the chip/encapsulant/discrete solder bump combination is placed on the substrate and subsequently heat is applied so that the solder is reflowed while simultaneously the encapsulant hardens, without the labor intensive underfill steps of the prior art.
In another aspect of the present invention there is provided a chip precoated with underfilling encapsulant having holes therein which expose metallized contact pads on the active surface of the chip. The holes are subsequently filled with molten solder which is then cooled and hardened to create the chip/encapsulant/discrete solder bump assembly. The assembly can be placed on a substrate and subsequently, the solder is reflowed while simultaneously the encapsulant hardens, eliminating the labor intensive underfill steps of the prior art. Alternatively, the chip/encapsulant/discrete solder bump assembly is coated with a thin layer of a flux adhesive and, subsequently, the solder is reflowed while simultaneously the flux adhesive and encapsulant harden.
In another aspect of the present invention there is provided a chip precoated with underfilling encapsulant having holes therein which expose metallized contact pads on the active surface of the chip. The holes are subsequently filled with an electrically conductive adhesive to create a chip/encapsulant/conductive adhesive bump assembly. The assembly can be placed on the substrate and subsequently the encapsulant and conductive adhesive are simultaneously hardened, without the labor intensive underfill steps of the prior art.
The present invention also provides a substrate precoated with the encapsulant having holes therein which expose the metallized solder pads on the substrate. The holes are subsequently filled with molten solder or electrically conductive adhesive which is then cooled and hardened prior to attachment of the chip to the substrate by reflow. In another embodiment, the substrate has encapsulant and separate discrete solder columns pre-assembled thereon.
In one aspect of the present invention, there is provided a first portion of an underfilling encapsulant and separate discrete solder bumps pre-coated and pre-assembled on a chip for assembly to a substrate. The first portion of encapsulant can be either a solid or a thick liquid, partially or fully uncured. A second portion of the encapsulant is applied to the substrate. The first portion of the encapsulant is filled, preferably highly filled, with a filler material to produce a reduced coefficient of thermal expansion and increased modulus. The second portion of the encapsulant is either lightly filled or completely devoid of filler material. At least the second portion of the encapsulant comprises an adhesive material with solder fluxing properties, for example, an adhesive flux. The first portion of the encapsulant can comprise a similar material or a conventional epoxy. The first portion is filled with a filler having a lower coefficient of thermal expansion and higher modulus than the encapsulant material without filler to increase the encapsulant""s modulus and reduce its coefficient of thermal expansion. The invention provides a simple, cost-effective assembly procedure wherein the chip/first portion of encapsulant/discrete solder bump combination is placed on the substrate/second portion of encapsulant combination and subsequently heat is applied so that the solder is reflowed while simultaneously the encapsulant cures, without the labor intensive, time-consuming underfill steps of the prior art. Preferably, the second portion constitutes a relatively thin layer in the overall encapsulant structure which somewhat intermixes with the first portion during cure and has minimal effect on the reliability of the flip-chip structure, despite the second portion having generally a lower modulus and higher coefficient of thermal expansion than the first portion. An advantage of the present invention is that the lower viscosity of the unfilled or lightly filled second portion during the reflow process allows the solder to flow without impediment from the thick viscosity of the first portion of the encapsulant. The present invention provides a low coefficient of thermal expansion and high modulus in the first portion of the encapsulant while at the same time achieving good solder wetting and chip self aligning in the second portion of the encapsulant.
In another aspect of the present invention, the chip/first portion of encapsulant/discrete solder bump assembly described above is coated with a thin layer of the second portion of the encapsulant which is either lightly filled or completely devoid of filler material. Placement of the chip, solder reflow and adhesive cure follows as described above.
In another aspect of the present invention, there is provided a method for placing a flip-chip onto a substrate that avoids entrapment of gas bubbles or creation of voids. The chip, having the first portion of encapsulant thereon, is oriented at an angle to the substrate having the second portion thereon, then pivoted about the first point of contact until the solder bumps on the chip are in contact with the solder pads on the substrate, creating an underfill of encapsulant material as the chip is pivoted while expelling the gas from between the chip and substrate.
Another aspect of the present invention provides a chip with underfilling encapsulant pre-coated and pre-assembled on the chip for assembly to a substrate, wherein the encapsulant consists of more than one layer, each layer performing one or more distinct functions such as attachment, stress distribution, electrical redistribution, reworkability, adhesion, or other functions. The bulk of the encapsulant, consisting of one or more layers, is applied and partially or fully hardened prior to assembly of the chip on the substrate. Holes therein which expose metallized contact pads on the active surface of the chip are subsequently filled with solder or an electrically conductive adhesive as previously described to create an encapsulated subassembly. Then a flux adhesive is applied between the chip/encapsulant/solder bump combination and the substrate which can be fully hardened after or when the chip/encapsulant/solder bump combination is placed on the substrate and the solder is reflowed.
Removal of the chip from the substrate is made possible by incorporating in the pre-coated multi-layer encapsulant a polymer layer that can be remelted even after the chip has been assembled to the substrate. Remelting the solder and the polymer encapsulant layer allows removal of the chip for repair or replacement after assembly or for test and burn-in of the chip prior to final assembly. Thus the-chip can be disassembled from the substrate without damage to either chip or substrate.
In another aspect of the present invention there is provided a redistribution of the chip""s electrical interconnection pads by incorporating in the pre-coated multilayer encapsulant an electrical redistribution layer comprising a thin printed circuit layer with electrical circuitry thereon. The interconnect pads on the chip are attached by solder bumps, conductive adhesive or wire bonds to the redistribution layer. The redistribution layer is subsequently encapsulated. Holes in the encapsulant expose metallized contact pads on the active surface of the redistribution layer. The holes are subsequently filled with solder as previously described. Then a flux adhesive layer is applied between the chip/encapsulant/redistribution layer subassembly and the substrate. The flux adhesive is applied remaining unhardened until the subassembly is placed on the substrate and the solder is reflowed.
Another aspect of the present invention also provides within the precoated encapsulant a novel compliant flexible structure wherein the solder and encapsulant expand or contract laterally without cracking or delaminating upon heating or cooling of the chip and substrate. The novel encapsulant mainly provides the adhesive mechanical bond required to hold the chip on the substrate while the solder mainly provides the electrical interconnection required between the chip and the substrate.
The compliant solder and flexible encapsulant of the present invention absorb the stress caused by the mismatched coefficients of thermal expansion without relying on bending of the chip and substrate. Since the mechanical adhesion of the chip to the substrate relies primarily on the encapsulant, a relatively soft, fatigue-less, highly pliable solder is used for the solder bumps to provide the electrical interconnection of the chip with the substrate. The compliant solder may have relatively weak mechanical properties on its own, therefore the encapsulant provides the mechanical strength. Relieving the solder of its mechanical tasks allows the use of soft, ductile and fluid-like solders that deform laterally with the expansion and contraction of the structure without the fatigue cracking normally experienced by conventional solders.
Another embodiment of the present invention also provides within the novel compliant encapsulant previously described a compliant conductive adhesive which expands or contracts laterally upon heating or cooling to absorb the stresses created by the mismatch in the coefficients of thermal expansion and prevent bending of the chip and substrate. Independent of each other, the structural properties of the novel encapsulant provides the mechanical connection required in the structure while the electrical properties of the compliant conductive adhesive provides the required electrical connection between the chip and the substrate.
The semiconductor chip package structures of the present invention provide, among other advantages, simple chip placement followed by reflow without labor intensive underfill steps; a solder bumped or conductive adhesive bumped chip or substrate with an encapsulant pre-attached, with the encapsulant performing a mechanical function and the solder or conductive adhesive performing an electrical function; a pre-coated chip encapsulant of two or more layers, each layer performing a distinct function of attachment or reworkability; a reworkable flip chip assembly by means of a remeltable polymer in the encapsulant; an electrical redistribution layer within the encapsulant; a low-cost method for applying the solder bumps to a flip chip or flip chip substrate by creating holes in a pre-coated encapsulant; and a low-cost method for applying the conductive adhesive bumps to a flip chip or substrate by creating holes in a pre-coated encapsulant; and a compliant chip understructure that includes a fatigue-less solder or conductive adhesive.